A silicon substrate in which a ferroelectric film has been formed on this kind of substrate body by a sol-gel process has heretofore been used for a composite electronic component such as a thin film capacitor, a capacitor, IPD (Integrated Passive Device), etc. In the method for forming the ferroelectric film on the above-mentioned conventional substrate body by the sol-gel process, however, there are problems that shrinkage of the film is generated at the time of forming the ferroelectric film on the substrate body, and the silicon substrate is markedly warped after formation of the ferroelectric film due to the difference in thermal expansion coefficients between the substrate body and the ferroelectric film. In addition, if the warpage of the silicon substrate is too remarkable, there is a problem that a thicker ferroelectric film cannot be formed since temperature unevenness is generated at the time of baking the ferroelectric film, whereby unevenness in residual stress is generated in the ferroelectric film.
To solve the above-mentioned problems, there has been disclosed a ferroelectric thin film in which an electrode layer (a first layer or an electric circuit layer) and a PZT layer (ferroelectric layer) are formed on the surface (one surface) of the substrate, and a stress balancing layer (a second layer or a stress relief layer) is formed on the back surface (the other surface) thereof (for example, see Patent Document 1.). In this ferroelectric thin film, a material made of Si on the surface of which has been formed a Si oxidized film is used as a substrate. The electrode layer comprises a base layer comprising Ti, and an electrode pattern layer comprising Pt formed on the base layer with a predetermined electrode pattern, and a thickness of the base layer is made to be about 50 nm and a thickness of the electrode pattern layer is made to be about 200 nm. As the base layer, TiO2 may be used. The PZT layer is formed by a material containing Pb, Zr and Ti, which is formed with a thickness of about 100 nm to 5 μm and in a crystallized state. The PZT layer can realize the above-mentioned thickness by laminating thin films each having 120 to 130 nm per layer with a plural number of layers.
For forming the ferroelectric thin film constituted as mentioned above, Ti which forms a base layer of the electrode layer and Pt which forms an electrode pattern layer are successively formed as films on the surface of a substrate (one surface) which had been formed by using the conventionally known Si substrate manufacturing techniques, etc., by the means of a sputtering method, etc. In addition, the stress balancing layer is also formed by forming Ti which forms a Ti layer and Pt which forms a Pt layer successively at the back surface (the other surface) of the substrate by the means of sputtering, etc. Then, the PZT layer is so formed that it covers the electrode layer. For forming the PZT layer, a sol-gel solution containing Pb, Zr and Ti with a predetermined formulation ratio is firstly coated on the electrode layer by means of a spin coating method, etc. Next, the sol-gel solution coated on the electrode layer is dried by holding it at 100 to 150° C. for 2 minutes, then, thermally decomposed by holding the same at 200 to 450° C. for 5 minutes, and further subjecting to heat treatment which holds at 550 to 800° C. for 1 to 10 minutes to crystallize the same. Thus, the PZT layer with a predetermined thickness is formed by subjecting to coating, drying, thermal decomposition and heat treatment of the sol-gel solution repeatedly with a plural number of times, and laminating a plural number of layers.
In the thus formed ferroelectric thin film, warpage of the ferroelectric thin film can be suppressed by providing a stress balancing layer (a second layer or a stress relief layer), since the residual stress of the electrode layer (a first layer or an electric circuit layer) and the PZT layer (ferroelectric layer) can be cancelled and relaxed. As a result, crack, peeling, etc., of the PZT layer (ferroelectric layer) can be prevented. More specifically, when the stress balancing layer is employed, the residual stress in the state of the ferroelectric thin film after forming the PZT layer can be suppressed to a lower range of 43.2 MPa to 139.3 MPa.